The present invention relates to floating gate nonvolatile memories.
A floating gate nonvolatile memory cell stores information by storing an electrical charge on its floating gate. The floating gate is capacitively coupled to the control gate. In order to write the cell, a potential difference is created between the control gate and some other region, for example, the source, drain or channel region of the cell. The voltage on the control gate is capacitively coupled to the floating gate, so a potential difference appears between the floating gate and the source, drain or channel region. This potential difference is used to change the charge on the floating gate.
In order to reduce the potential difference that has to be provided between the control gate and the source, drain or channel region, it is desirable to increase the capacitance between the control and floating gates relative to the capacitance between the floating gate and the source, drain or channel region. More particularly, it is desirable to increase the “gate coupling ratio” GCR defined as CCG/(CCG+CSDC) where CCG is the capacitance between the control and floating gates and CSDC is the capacitance between the floating gate and the source, drain or channel region. One method for increasing this ratio is to form spacers on the floating gate. See U.S. Pat. No. 6,200,856 issued Mar. 13, 2001 to Chen, entitled “Method of Fabricating Self-Aligned Stacked Gate Flash Memory Cell”, incorporated herein by reference. In that patent, the memory is fabricated as follows. Silicon substrate 104 (FIG. 1) is oxidized to form a pad oxide layer 110. Silicon nitride 120 is formed on oxide 110 and patterned to define isolation trenches 130. Oxide 10 and substrate 104 are etched, and the trenches are formed. Dielectric 210 (FIG. 2), for example, borophosphosilicate glass, is deposited over the structure to fill the trenches, and is planarized by chemical mechanical polishing (CMP). The top surface of dielectric 210 becomes even with the top surface of nitride 120. Then nitride 120 is removed (FIG. 3). Oxide 110 is also removed, and gate oxide 310 is thermally grown on substrate 104 between the isolation trenches. Doped polysilicon layer 410.1 (FIG. 4) is deposited over the structure to fill the recessed areas between the isolation regions 210. Layer 410.1 is polished by chemical mechanical polishing so that the top surface of layer 410.1 becomes even with the top surface of dielectric 210.
Dielectric 210 is etched to partially expose the “edges” of polysilicon layer 410.1 (FIG. 5). Then doped polysilicon 410.2 (FIG. 6) is deposited and etched anisotropically to form spacers on the edges of polysilicon 410.1. Layers 410.1, 410.2 provide the floating gates.
As shown in FIG. 7, dielectric 710 (oxide/nitride/oxide) is formed on polysilicon 410.1, 410.2. Doped polysilicon layer 720 is deposited on dielectric 710 and patterned to provide the control gates.
Spacers 410.2 increase the capacitance between the floating and control gates by more than the capacitance between the floating gates and substrate 104, so the gate coupling ratio is increased.